The invention relates to a structured packing for heat exchange and mass transfer between a liquid and a gas in a column. For heat exchange and mass transfer between liquid and gaseous media, in particular for the separation of mixtures by distillation, plate columns and packed columns are used in industry. The two types differ with respect to the hydrodynamic operating conditions.
In the case of plate columns, in each case a bubbling layer forms on the individual plates where predominantly the liquid is the continuous phase and the gas the disperse phase. Between the individual plates are free spaces in which predominantly the gas is the continuous phase.
The mode of operation of packed columns differs from plate columns with respect to hydrodynamics. In this case it is not the liquid but the gas which forms the continuous phase. The liquid runs as a film downward over the packings.
Structured packings are made up of a multiplicity of individual layers of packing elements, such as metal sheets, expanded metals and wire fabrics, which are disposed vertically to one another in a regular structure and are usually held together in a composite by attachments such as metal wires, thin metal rods or metal sheet strips. Usually the packing elements themselves have a geometric structuring, for example in the form of folds or circular holes of from about 4 to 6 mm in diameter. The openings act to increase the flood limit of the packing and to make a higher column load possible.
Examples are packings of the types xe2x80x9cMellapakxe2x80x9d, CY and BX from Sulzer AG, CH-8404 Winterthur, or types A3, BSH or B1 from Montz GmbH, D-40723 Hilden. The folds of the packing elements of these packings run linearly and at an angle of from about 30xc2x0 to 45xc2x0 to the longitudinal axis of the packing. The foldings of the packing elements lead to a cross-channel structure within the structured packing.
DE 196 05 286 A1 describes a special development in which this angle is further decreased to values of from 3xc2x0 to 14xc2x0 in order to reduce the pressure drop of the packings as far as possible in the case of applications at high vacuum (approximately 1 mbar top pressure).
In the prior art, structured packings are known which are catalytically active. A catalytically active distillation packing in a conventional shaping is, for example, the packing xe2x80x9cKATAPAKxe2x80x9d from Sulzer AG, CH-8404 Winterthur.
Structured packings are usually provided as individual packing layers which are then arranged in the column stacked one above the other. The packing layers usually have a height of from about 0.17 m to about 0.30 m.
In the prior art, a structured packing called xe2x80x9cMontzxe2x80x9d A2 from Montz GmbH, D-40723 Hilden is known, which has folded packing elements with curved fold courses. Within a packing element, the gradient of these fold courses varies over the height of the packing element. In this case the layers of the packing elements alternate so that in each case one packing element in which the gradient of the fold line is greatest at the bottom end of the packing layer alternates with a packing element in which the gradient of the fold line is greatest at the top end of the packing layer. The internal geometry of the packing layer is therefore constant over its height. However, this packing type, in comparison with the usual structured packings, has an unfavorable separation efficiency.
Because of the industrial importance of heat exchange and mass transfer processes in chemistry and process engineering, in particular separation by distillation, a multiplicity of technical developments are aimed at improving heat exchange and mass transfer columns, in particular distillation columns. Important criteria for an efficient economic heat exchange and mass transfer column, in particular distillation column, are its price, its throughput performance for the gas and liquid stream and the separation efficiency based on the height of the column. It is usually characterized as the number of theoretical plates per meter of column height (nth/m) or as the height equivalent to a theoretical plate (HETP).
It is an object of the present invention to increase the throughput and economic efficiency of heat exchange and mass transfer columns, in particular for distillation purposes.
We have found that this object is achieved by a structured packing for heat exchange and mass transfer between a liquid and a gas in a column having at least one packing layer with a first, lower end and a second, upper end, the packing layer having an internal geometry which varies over its height so that by suitably setting the liquid and gas flow rates in a first, in particular lower, region of the packing layer a bubbling layer having a predominantly disperse gas phase forms in a targeted manner and simultaneously in a second, in particular upper, region of the packing layer a film flow of the liquid having a predominantly continuous gas phase forms in a targeted manner.
The internal geometry is therefore, in contrast to structured packings of the prior art, not constant over the height of the packing layer.
The hydrodynamic operating states described can be achieved by the resistance to flow varying over the height of the packing layer. Preferably the first, optionally lower, region of the packing layer has a higher resistance to flow than the second, optionally upper, region of the packing layer.
The first region of the packing layer is preferably situated in a lower region of the packing layer and the second region of the packing layer is preferably in an upper region of the packing layer. For the purposes of the present invention, the first, optionally lower, region and the second, optionally upper, region of the packing layer preferably extend over the entire cross-sectional area of the packing layer. The first, lower, region of the packing layer can be bound directly to the lower end of the packing layer and the second, upper region of the packing layer can be bound directly to the upper end of the packing layer. In a preferred embodiment, the first, optionally lower, region of the packing layer is connected directly to the second, optionally upper, region.
In the context of the present invention a structured packing is preferred in which the packing layer has touching flat packing elements, in particular metal sheets, expanded metals, wire fabrics and knitted meshes, having folds of defined courses, the fold courses or tangents to the fold courses being at a larger angle to the longitudinal axis of the packing layer in the first region of the packing layer than in the second region of the packing layer. Particularly preferably, the fold courses or the tangents to the fold courses of the packing elements are at an angle of from about 45xc2x0 to about 75xc2x0 to the longitudinal axis of the packing layer in the first region of the packing layer and from about 10xc2x0 to about 45xc2x0 in the second region. Very particularly preferably, the fold courses or the tangents to the fold courses are at an angle of from about 60xc2x0 to about 70xc2x0 to the longitudinal axis of the packing layer in the first region of the packing layer and from about 30xc2x0 to about 45xc2x0 in the second region.
The folds can have, at least in sections, a curved or linear course.
In a preferred embodiment, the folds are curved in a shape of monotonic course, so that the tangents to the fold courses are at an angle of from about 45xc2x0 to about 75xc2x0, preferably from about 60xc2x0 to about 70xc2x0, to the longitudinal axis of the packing layer at the lower end of the packing layer, this angle of the tangents to the fold courses decreasing upwardly to values of from about 10xc2x0 to about 45xc2x0, preferably from about 30xc2x0 to about 45xc2x0, to the longitudinal axis of the packing layer.
The structured packing can also be designed such that the fold courses are linear in sections, the fold courses preferably being at an angle of from about 45xc2x0 to about 75xc2x0, particularly preferably from about 60xc2x0 to 70xc2x0 to the longitudinal axis of the packing layer in the first region of the packing layer and the angle of the fold courses to the longitudinal axis of the packing layer decreasing upwardly in one step or in a plurality of steps to values of, preferably from about 10xc2x0 to about 45xc2x0, particularly preferably from about 30xc2x0 to about 45xc2x0.
The specific surface area of the inventive structured packings is preferably from about 100 to 750 m2/m3, particularly preferably from 250 to 500 m2/m3.
The folds in the packing elements can be made with sharp edges or rounded.
The first region of the packing layer preferably has a height of from 0.02 to 0.10 m, more preferably from 0.03 to 0.10 m, and particularly preferably from 0.03 to 0.05 m.
The second region of the packing layer preferably has a height of from 0.1 to 0.3 m, and particularly preferably from 0.15 to 0.2 m.
The packing layers of the inventive structured packing preferably have a height of from 0.05 to 0.40 m, more preferably from 0.08 to 0.35 m or from 0.10 to 0.25 m, and particularly preferably from 0.12 to 0.20 m. The lower height of the packing layer is preferably provided for closely packed packings having a specific surface area of from about 500 to about 750 m2/m3, the higher value for coarser packings having from about 100 to about 500 m2/m3.
The liquid load of the structured packing is preferably from about 0.2 to 50 m3/m2h. At from 70 to 80% of the flood limit, the pressure drop of the inventive packing is preferably from about 2 to 10 mbar/m. The packing elements preferably have a metal sheet thickness of about 0.1 mm.
In a preferred embodiment of the present invention in which the packing layer has packing elements, at least some of the packing elements are bent over in tongue-like manner at the lower end and/or upper end of the packing layer. Preferably, the packing elements have cuts for this at the lower end and/or upper end of the packing layer at defined distances which preferably correspond to about half the fold width, so that tongues can be bent over in different directions. Particularly preferably, the tongues are bent over alternately toward both sides of the packing element. The depth of the cuts is preferably from 3 to 8 mm. The angle which the bent-over tongues make with the packing element is preferably from about 110 to 150xc2x0, so that the tongues are roughly horizontally oriented in the packing layer. The lateral extension of the tongues is chosen so that from about 30 to 60% of the flow cross section is blocked. Preferably, only every second sequential packing element is bent over laterally in order to ensure sufficient mechanical stability of the packing layers stacked one above the other.
In a further preferred embodiment of the present invention in which the packing layer also has packing elements, strips preferably made of sheet metal are disposed between at least some of the packing elements. These are preferably made planar. The strips are preferably situated at the lower end of the packing layer. They can be disposed unilaterally or bilaterally on the packing elements and are preferably attached to these. Particularly preferably, the strips are attached to the packing elements by point welding. The strips preferably have a height of from about 15 to 25 mm. One end of the strips, preferably the upper end of the strips, is preferably bent over at the side by from about 2 to about 5 mm. This further advantageously increases the resistance to flow. The lateral bending of the strips is situated preferably between the folds of the packing elements. The lateral bending of the strips can take place during the assembly of the packing elements to form a packing layer.
The present invention comprises a further preferred embodiment in which the packing layer is composed of a combination of at least one first partial packing layer and one second partial packing layer, the first partial packing layer and the second partial packing layer differing from one another with respect to their internal geometries. In this case, in the first packing layer, the first partial packing layer is preferably disposed underneath the second partial packing layer. Particularly preferably, the first partial packing layer and the second partial packing layer are disposed directly one over the other, the first partial packing layer forming the lower partial packing layer and the second partial packing layer forming the upper partial packing layer. The partial packing layers are preferably designed so that their internal geometry does not vary over their height. The first, optionally lower, partial packing layer preferably has a height of from 0.02 to 0.10 m, and particularly preferably from 0.03 to 0.05 m. The second, optionally upper, partial packing layer preferably has a height of from 0.05 to 0.40 m, particularly preferably from 0.10 to 0.25 m. The resistance to flow of the first partial packing layer per meter height is preferably from about 1.2 to about 5 times, particularly preferably from about 1.5 to about 2.5 times, as high as the resistance to flow of the second partial packing layer. If the partial packing layers are composed of packing elements with folds, the resistance to flow of the partial packing layers can be set by the angle which the fold courses or tangents to the fold courses make with the longitudinal axis of the packing layer. The larger is this angle, the higher is the resistance to flow. In the context of the present invention an embodiment is preferred in which the partial packing layers are composed of packing elements with folds, the fold courses or tangents to the fold courses of the first partial packing layer being at a greater angle to the longitudinal axis of the packing layer than the fold courses or tangents to the fold courses of the second partial packing layer. Preferred angles have already been mentioned above, which are here incorporated by reference. The abovementioned first region of the packing layer corresponds in this case to the first partial packing layer mentioned here and the abovementioned second region of the packing layer corresponds to the second partial packing layer mentioned here. The resistance to flow of the partial packing layers can, furthermore, also be achieved by the size of the specific packing surface area per unit volume. Preferably, the partial packing layers have different specific surface areas per unit volume. Particularly preferably, the first, optionally lower, partial packing layer has a higher specific surface area per unit volume than the second, optionally upper, partial packing layer. In this case the specific surface area of the first, optionally lower, partial packing layer is preferably from 20 to 100%, particularly preferably from 30 to 60%, greater than that of the second, optionally upper, packing layer. In a particularly preferred embodiment, the first, optionally lower, partial packing layer is made of wire meshes. This makes it possible to set the liquid contents in a targeted manner by changing the heating power. The partial packing layers are preferably disposed twisted round from one another by from 45xc2x0 to 90xc2x0.
The packing elements can have a thin coating of noble metal catalysts. This plays a role if, in a column having the inventive packing, in addition to the heat exchange and/or mass transfer, catalyzed reactions are also intended to proceed.
In the context of the present invention, therefore, furthermore, a process is provided for heat exchange and/or mass transfer between a liquid and a gas in a column, in which the liquid and the gas are conducted via an above-described structured packing, in particular in countercurrent flow, and the liquid and gas flow rates are set such that in a first, in particular lower, region of the packing layer, a bubbling layer having a predominantly disperse gas phase forms in a targeted manner and, simultaneously, in a second, in particular upper, region of the packing layer, a film flow of the liquid having a predominantly continuous gas phase forms in a targeted manner.
The column is preferably operated at a pressure drop of from about 5 to about 30 mbar, particularly preferably from about 8 to 12 mbar, per meter of packing height. The pressure drop can be set via the liquid and gas flow rates and by the heating power.
In a particular embodiment, superimposed on a separation by distillation, a chemical reaction proceeds in the column. It can be catalyzed homogeneously or heterogeneously or proceed spontaneously. The residence time of the liquid in the column can be set in a targeted manner by selecting the heating power with the differential pressure being measured.
Homogeneously catalyzed reactions can be, for example, acid catalyzed acetalizations, acetal cleavages, esterifications, saponifications and ether formations, and also alkoxide-catalyzed transesterifications. An example of a spontaneously proceeding reaction in a distillation column is the separation of formaldehyde from aqueous or alcoholic solutions.
It is also possible to coat the inventive structured packings with catalytically active material directly using processes which are already established in the art than to carry out heterogeneously catalyzed reactive distillations. If, for reasons of cost, only parts of the packings are to be coated with catalytically active material, it is expedient to coat preferably the first, optionally lower, region of the packing layer, in which predominantly the bubbling layer forms, since here particularly good mass transfer conditions occur.
The inventive packings are also suitable for reactive distillations in which the packings are coated with a thin layer of noble metal catalysts. In this case, partial hydrogenations can then be carried out in the presence of hydrogen. Particularly preferably, from a C4 hydrocarbon mixture, components with triple bonds are highly selectively hydrogenated to form components with double bonds using the present process at a total pressure of from 3 to 8 bar, particularly preferably about 4 bar.
The present invention essentially has the following advantages:
The separation efficiency of the inventive structured packing is, when the described hydrodynamic state is established, up to 60% higher than the separation efficiency of structured packings according to the prior art. As a result, the required column height can be decreased and thus capital costs be saved.
The separation efficiency of the column is customarily characterized as a number of theoretical plates per meter of column height (nth/m) or as height equivalent to a theoretical plate (HETP).
The inventive packings, due to the high liquid hold up in the packing, also open up other fields of application which have been reserved hitherto for plate columns or special constructions. Thus, some chemical reactions may be carried out particularly advantageously in columns which are equipped with the inventive structured packings.
The structured packing designed and operated according to the invention is a transition form between a packed column with a predominantly disperse liquid phase and a plate column with a predominantly continuous liquid phase. Favorable properties of a plate column (high mass transfer performance in the bubbling layer) and a packed column (prevention of drop entrainment and additional mass transfer at the packing surface) can thus be combined.
Other advantages, features and potential uses of the invention will now be described in detail on the basis of examples with reference to the accompanying drawing.